The following is courtesy of Children's Hospital Boston, Mass General Hospital for Children and the National Institutes of Health

The diaphragm is the structure that separates the thoracic and abdominal cavities to maintain the pressure differentials of the respective compartments.

Congenital diaphragmatic hernia (CDH) refers to a developmental defect of the formation of the diaphragm that, in most cases, is evident at birth. Although CDH is classified into several types, distinction among hernias can be problematic. An anatomic depiction of the normal diaphragm is presented in Figure 1 and anatomic descriptions of diaphragmatic defects are presented in Figure 2

Posterolateral (Bochdalek) hernia. This posterolateral defect in the diaphragm, commonly referred to as a Bochdalek hernia, is often accompanied by herniation of the stomach, intestines, liver, and/or spleen into the chest cavity. An extremely large defect, or apparent absence of the hemidiaphragm, is called agenesis of the diaphragm; this defect probably represents the severe end of the Bochdalek hernia spectrum rather than a distinct entity.

Posterolateral hernias comprise approximately 80%-90% of all CDH and appear to fall into two types: a) a diaphragmatic defect accompanied by absent or extremely deficient rim of posterior and lateral musculature (see Figure 2a); and b) a diaphragmatic defect with an intact rim of posterior and lateral musculature.

About 85% of Bochdalek hernias occur on the left side, about 10% on the right, and approximately 5% are bilateral.

Non-posterolateral (non-Bochdalek) hernia. Anterior defects of the diaphragm can occur in the midline, on the left side, or the right side. Several distinct "subtypes" are described, but considerable overlap in the anatomic location among these defects exists. Furthermore, it is not clear whether these "subtypes" share a common embryologic mechanism.

• a
  Morgagni (Morgagni-Larrey) hernia. Morgagni hernia is an anterior retrosternal or parasternal hernia that can result in the herniation of liver or intestines into the chest cavity (see Figure 2b). Morgagni hernias comprise approximately 2% of all CDH, are generally accompanied by a hernia sac, and often do not cause symptoms in the newborn period.
  

• b

  Other anterior hernias associated with Pentalogy of Cantrell. These rare and severe types of hernias, possibly derived from the septum transversum, are found in individuals with Pentalogy of Cantrell (which also includes defects in the supraumbilical midline abdominal wall, lower sternum, diaphragmatic pericardium, and heart).

• c
  Central hernia. This hernia is a rare diaphragm defect involving the central tendinous (e.g., amuscular) portion of the diaphragm. The entire rim of diaphragmatic musculature is present (see Figure 2c). Whether abnormal septum transversum development results in central as well as more anteriorly located hernias, and also whether central hernias can be distinguished from posterolateral hernias with a complete rim of musculature, is still a matter of debate.

Diaphragmatic eventration. Diaphragmatic eventration is incomplete muscularization of the diaphragm resulting in a thin membranous sheet of tissue. It is difficult to estimate the frequency of eventration because it may co-exist with a Bochdalek hernia with which it is often clinically misdiagnosed. Severe diaphragmatic eventration is associated with pulmonary hypoplasia and respiratory distress during infancy. Milder degrees of diaphragmatic eventration can present later in life with respiratory symptoms such cough and pneumonias or without symptoms so that the diagnosis is made incidentally on chest x-ray. Increasingly, it is observed that eventration of the diaphragm and "true" CDH can occur in the same individual, suggesting that in some instances they share a common etiology.

All hernia types can present with a sac (e.g., membranous sheet of tissue) covering the herniated abdominal contents. There currently is no explanation for the development of a hernia sac; however, its presence portends a better prognosis. Because a thin and redundant membranous diaphragm resulting from an eventration defect may represent a "sac," it is probable that eventration and "sac type" CDH diagnoses are often interchanged.

Embryology. Development of the diaphragm takes place between the fourth and twelfth weeks of pregnancy. The normal development of the diaphragm is not well understood. The diaphragm forms as a membranous sheet that most likely arises from different components:

• The central portion and possibly anterior regions of the diaphragm are thought to develop from the septum transversum, which is initially fused to the liver during development and becomes the unmuscularized central tendon of the diaphragm [Yuan et al 2003]. The contribution to the mature diaphragm from the septum transversum remains poorly understood.

• The posterolateral section of the diaphragm , the region that is deficient in the Bochdalek hernia, is thought to develop, in part, from the pleuroperitoneal folds (PPFs), triangular structures derived from mesoderm that develop in the thorax during early diaphragmatic development. These PPFs are thought to contribute to the connective tissue portion of the diaphragm. The membranous diaphragm is subsequently muscularized by muscle precursor cells that migrate first to the PPF from the cervical somites before proliferating, differentiating, and migrating onto the membranous diaphragm [Birchmeier & Brohmann 2000, Babiuk & Greer 2002, Babiuk et al 2003]. It is believed that a Bochdalek hernia forms if the PPFs do not fuse with the septum transversum and the dorsal mesentery of the esophagus by the tenth week of gestation

• Co-existing pulmonary abnormalities. The pathogenesis of the pulmonary hypoplasia so frequently associated with CDH is not fully known, but appears to have both a primary component, i.e., the hypoplasia occurs along with the diaphragm defect, and a secondary component, i.e., arising from competition for thoracic space particularly in the lung ipsilateral to the hernia. Differing degrees of bilateral pulmonary hypoplasia may explain the variance in severity seen among neonates presenting with respiratory distress and CDH. This is supported by the nitrofen and Fog2Management, Animal models) in which both primary pulmonary hypoplasia and diaphragmatic defects are observed [Guilbert et al 2000, Ackerman et al 2005]. animal models (see

• Abnormal pulmonary vascular development and function is a significant problem in infants with CDH. The mechanism of pulmonary hypertension in CDH is not completely understood. The size of the pulmonary vascular bed is decreased in the hypoplastic lungs, and the adventitia and media of the pulmonary arterial walls are thickened [Yamataka & Puri 1997]. These changes occur in utero and are more severe in term neonates than in pre-term neonates.

Taken from CHERUBS membership data in 2009 involving 2432 CDH patients: